Public Release: 16-Jul-2012
AGU Journal highlights -- July 16

The following highlights summarize research papers that have been recently
published in Journal of Geophysical Research-Biogeosciences (JGR-G), Journal
of Geophysical Research-Atmospheres (JGR-D), Journal of Geophysical
Research-Planets (JGR-E), Geochemistry, Geophysics, Geosystems (G3), and
Geophysical Research Letters (GRL).

1. Droughts threaten Bornean rainforests

At 130 million years old, the rainforests of Southeast Asia are the oldest in the world
and home to thousands of plant and animal species, some endemic to these forests.
The rainforests also play important roles in modulating regional rainfall as well in the
global carbon cycle.

However, since the 1960s, increased warming in the Indian Ocean and frequent El
Nino events have reduced rainfall in the region by approximately 1 percent per
decade. Further, the Intergovernmental Panel for Climate Change predicts that over
the 21st century, Southeast Asia will experience higher land temperatures, more
droughts, and increased seasonality -- wet seasons during the fall will get wetter, and
dry seasons during the spring will get drier. However, few studies in the past have
investigated how trees in the southeastern Asian rainforests respond to droughts and
climate change.

In a new study, Kumagai and Porporato combine extensive field observations,
historical records, and global climate models to investigate the potential impact of
rainfall shifts and droughts on tree mortality in the Bornean rainforests of Southeast
Asia. They find that as El Nino events become more frequent in the future in
response to warming in the tropical oceans, even the species of trees that can adapt to
drought conditions will be at increased risk of dying off. The small number of
species that cannot adapt well to drought conditions will be at even greater risk of
dying off.

Their study has implications for predictions of ecological changes, regional rainfall
patterns, and global climate as well as direct applications for policies aimed at
reducing additional human impacts on these ecosystems, which are not only
vulnerable to climate change but also have the highest rates of deforestation in the
whole world.

A debate has raged in the past couple of years as to whether natural gas is better
or worse overall than coal and oil from a global warming perspective. The back-
and-forth findings have been due to the timelines taken into consideration, the
details of natural gas extraction, and the electricity-generating efficiency of
various fuels. An analysis by Cathles, which focuses exclusively on potential
warming and ignores secondary considerations, such as economic, political, or
other environmental concerns, finds that natural gas is better for electricity
generation than coal and oil under all realistic circumstances.

To come to this conclusion, the author considered three different future fuel
consumption scenarios: (1) a business-as-usual case, which sees energy
generation capacity continue at its current pace with its current energy mix until
the middle of the century, at which point the implementation of low-carbon
energy sources dominates and fossil fuel-derived energy production declines; (2)
a gas substitution scenario, where natural gas replaces all coal power production
and any new oil-powered facilities, with the same midcentury shift; and (3) a low-
carbon scenario, where all electricity generation is immediately and aggressively
switched to non-fossil fuel sources such as solar, wind, and nuclear.

The author finds that the gas substitution scenario would realize 40 percent of the
reduction in global warming that could be achieved with a full switch to low-
carbon fuel sources. The benefit for mitigating warming revolves around the fact
that to produce an equivalent amount of electricity burning natural gas would
release less carbon dioxide than burning oil or coal. Though atmospheric methane
traps more outgoing radiation than carbon dioxide does, at reasonable leakage
rates its atmospheric concentration is much lower and what is released
decomposes much more quickly. The author suggests that over timescales
relevant to large-scale warming-decades to centuries-the effect of any methane
released during natural gas extraction would be inconsequential.

In December 1972 the astronauts of Apollo 17-the last manned mission to the
moon-deployed the Lunar Atmospheric Composition Experiment (LACE), a
spectrometer designed to measure and characterize the thin lunar atmosphere.
Forty years later, Stern et al. built upon those initial measurements, providing the
first remotely-sensed measurement of the Moon's gaseous environment from
lunar orbit. Using the Lyman Alpha Mapping Project's (LAMP's) far ultraviolet
spectrograph aboard the Lunar Reconnaissance Orbiter, the authors determined
the atmospheric concentration of helium.

By angling LAMP's sensors towards the lunar limb and comparing those
observations against measurements of the interstellar background, the authors
were able to estimate the helium concentration of the near-surface lunar
environment. They calculate a density of 7,000 atoms per cubic centimeter at 120
degrees Kelvin (-244 degrees Fahrenheit), the assumed atmospheric temperature.
The previous LACE observations ranged between 10,000 - 20,000 and 50,000
atoms per cubic centimeter depending on the time of day, increasing at nighttime
and decreasing during the day. The nighttime decrease occurs because the
atmosphere cools and contracts, yielding an increased density.

The authors suggest that the next steps should involve looking for spatial or
temporal variations in lunar atmospheric helium. Such observations could help to
determine whether the helium is produced locally by radioactive decay of lunar
material or if it is formed from trapped and neutralized solar wind.

The Atlantic Meridional Overturning Circulation (AMOC), driven by temperature
and salinity gradients, is an important component of the climate system; it
transfers an enormous amount of heat via ocean currents and atmospheric
circulation to high northern latitudes and hence has bearing on climate in the
region.

Freshening of the surface ocean could weaken the AMOC. But during warm
interglacial periods the effect of a fresh surface ocean on the AMOC may be
muted. In fact, climate models predict that heat transfer from the North Atlantic to
the Arctic may increase over the 21st century. A series of interconnected
processes in the North Atlantic, known as polar amplification, could cause the
Arctic to warm up faster compared to the rest of the world. It could even lead to
ice-free conditions in the Arctic.

Previous paleoclimatic reconstructions indicate that the sub-Arctic may have been
warmer by about 5 degrees Celcius (9 degrees Fahrenheit) with little summer sea
ice cover during the Eemian, the penultimate interglacial centered around 125,000
years ago. Climate models favoring polar amplification use the Eemian as an
analog of the present. In a new study, Bauch et al. compare reconstructed
temperatures and water masses from two sediment cores that record the flow of
meltwater in the subpolar and polar North Atlantic over the past 135,000 years.
They do not find evidence of extreme warmth in the sub-Arctic during the Eemian
interglacial period.

In fact, the Arctic may have been colder during the Eemian, with lower heat
transfer from the North Atlantic. On the basis of their finding, the authors suggest
that previous records may reflect other phenomena and caution against the use of
the Eemian as an analog of the present. Their finding also challenges climate
models that predict extreme warmth and ice-free conditions in the Arctic in
response to greenhouse gas warming in the 21st century.

Forest fires and emission of air pollutants, which include fumes from vehicles
running on diesel and slow burning of coal and charcoal, release isocyanic acid in
the troposphere. In 2011, scientists first detected isocyanic acid in the ambient
atmosphere at levels that are toxic to human populations; at concentrations
exceeding 1 parts-per-billion by volume (ppbv), human beings could experience
tissue decay when exposed to the toxin.

For the first time, using a chemical transport model designed to estimate the
distribution and budget of isocyanic acid in the troposphere, Young et al. show
that in several parts of the world, local emissions may increase the concentration
of isocyanic acid in ambient atmosphere, thereby exposing large populations to
potentially toxic levels of the acid.

Their research shows that regions that experience large forest fires, such as
tropical Africa, Southeast Asia, Siberia, Canada, and the Amazon, or are heavily
polluted, like China, are particularly vulnerable. In these regions, concentrations
of isocyanic acid in the atmosphere exceeded the 1 ppbv limit for about 7-90 days
per year. Their model also predicts that doubling the rate of air pollutant emission,
particularly in heavily polluted regions of China, could increase the exposure of
humans in the region to more than 170 days per year to isocyanic acid levels
exceeding 1 ppbv.

On the basis of their study, the authors recommend more observations to improve
estimates on global distribution of isocyanic acid in the atmosphere, particularly
in regions experiencing large wild fires, where their model predicts the highest
acid concentrations. Further, the authors suggest that scientists need to conduct
research into indoor air pollution from the use of cooking stoves, which likely
expose women and children to high levels of isocyanic acid.

The lunar surface is marred by impact craters, remnants of the collisions that have
occurred over the past 4.5 billion years. The Orientale basin, the Moon's most
recently formed sizeable crater, stands out from the rest. The crater, which lies
along the southwestern boundary between the near and far sides of the moon,
appears as a dark spot ringed by concentric circles of ejecta that reach more than
900 kilometers (560 miles) from the impact location. Though other craters have
similar rings, the lunar surface surrounding the Orientale basin is unusually rough
with reduced concavity. The anomalous features were identified by Kreslavsky
and Head after they produced a map of the lunar surface topographic roughness
using observations from the Lunar Orbiter Laser Altimeter aboard the Lunar
Reconnaissance Orbiter.

The fact that other craters-even those of similar size and age-lack similar
features suggests to the authors that mechanisms such as weathering or
gravitational settling cannot explain the anomaly. Instead, the authors suggest that
the Orientale basin, which formed about 3.8 billion years ago, stands out simply
because it is the youngest large crater. They propose that whenever a large body
slams into the Moon, seismic waves produced during the impact travel through
the solid lunar material, inducing seismic shaking that causes landslides and
surface settling. They estimate that the impactor would need to be at least 100 km
(62 mi) across to cause sizeable seismic shaking. Unfortunately, the authors may
need to wait more than a little while to conclusively test their hypothesis-until
the Moon is next rocked by a massive asteroid, an event not expected to occur in
the foreseeable future.

Anyone may read the scientific abstract for any already-published paper by
clicking on the link provided at the end of each Highlight. You can also read the
abstract by going to http://www.agu.org/pubs/search_options.shtml and inserting
into the search engine the full doi (digital object identifier), e.g.
10.1029/2011JG001835. The doi is found at the end of each Highlight below.

Journalists and public information officers (PIOs) at educational or scientific
institutions who are registered with AGU also may download papers cited in this
release by clicking on the links below. Instructions for members of the news
media, PIOs, and the public for downloading or ordering the full text of any
research paper summarized below are available at
http://www.agu.org/news/press/papers.shtml.

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